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Electrical
Test Parameter Cpk

What does the Cpk number for an electrical test parameter stand for?

The
calculated Cpk for an electrical test parameter has the same meaning as
the Cpk values calculated for the more commonly-encountered measured
quantities that are physical in nature. For a definition and brief
explanation of what Cpk is, please click here.

Cpk is
basically a measure of how well the output of your process meets its
lower and upper specifications. It is only applicable when the
output response exhibits a normal distribution. Cpk measures two
things: 1) how close the mean of the readings are to the center of the
lower and upper spec limits (ideally, the mean of the readings must
equal the center of the spec limits); and 2) how widely spread the
readings are (ideally, the standard deviation of the readings should be
zero) . The higher the Cpk, the better is the capability of the
process to meet its requirements.

In the
context of electrical parameter testing, the same Cpk calculation
concepts apply. Thus, Cpk may only be computed in test parameters that
have actual numerical measurements that form a normal distribution, and
the measurement readings should come from a statistically significant
number of sample units belonging to a homogeneous population.

Consider a
test parameter that meets the requirements for Cpk computation. A
certain number of units are tested for this parameter, and all the
measurement data are then collected. The mean and standard
deviation of the data are then used to compute for the Cpk of that test
parameter, using the same Cpk equation shown in the link above.

As mentioned,
the higher the computed Cpk is, the better. In the semiconductor
industry, a Cpk of less than 1.66 needs a closer look. A Cpk
that's less than 1.33 needs some action to make it higher, and a Cpk of
less than 1.0 means that the process is not capable of meeting its
requirements, and therefore requires an even more urgent response.

The lower the Cpk
of
an electrical test parameter, the higher is the probability that a
device from the same process will fail this parameter.
When analyzing a Cpk, it is important to determine if: 1) the mean of
the readings are close to or far from the center of the specs; and 2)
the standard deviation of the readings are low or high. A low Cpk
means one of 3 things: the mean is far from the center of the specs, or
the standard deviation of the readings is high (i.e., the readings are
widely spread), or both conditions exist.

The
corrective actions needed to move the readings closer to the center of
the specs are generally different from the corrective actions needed to
reduce the standard deviation of the readings. In any case, the test
engineer must understand why the readings behave in a certain way, and
define the corrective actions needed to rectify the situation.

Possible
corrective actions include but are not limited to: exclusion of
invalid data that are proven to be due to unrelated assignable causes,
repair or substitution of defective test hardware or software used in
the test, redesign of the test hardware and software to make it more
stable and accurate; redefinition of the lower and upper spec
limits of the test parameter based on the actual capability of the
device and test system; or even redesign of the device circuit and
packaging itself. Note that it may be necessary to collect a
new set of data to recalculate the Cpk of a test parameter if the data
initially collected are proven to be a misrepresentation of the actual
process capability.

See Also:
Electrical Testing

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